The invention relates to an acoustic surface-wave filter comprising at least one element which can be excited to perform surface oscillations, which is made at least partly of piezoelectric material and the surface of which is provided, on at least one side thereof, with interference locations for surface waves, the filter also comprising means for converting electrical into mechanical energy and vice versa.
Acoustic surface-wave filters have a transmission behaviour such that, without increasing the dimensions of the transducer fingers, the attenuation frequency substantially follows the curve (sin x/x).sup.2, where x is a linear function of the frequency. This behaviour, with regard to the flank slope, is approximately the same as that of a three-circuit band-pass filter. There are known methods of influencing this transmission behaviour, either by increasing the dimensions of the transducer fingers or by constructing coupled resonance structures in the path of the surface wave, so that the filter flanks can be made steeper in a manner corresponding to a multi-circuit band-pass filter. The transducer fingers can be enlarged e.g. by varying the width or length of the electrodes, as is disclosed by the article "Akustische Oberflachenwellen-Filter" by R. F. Mitchell in "Philips techn. Rundschau," 32, 1971/72, Nos 6/7/8, pages 191-202. Varying the width of the electrodes has the advantage of reducing diffraction effects to a minimum. Its disadvantage is that an accurate photo-etching technique is required. Varying the length of the electrodes, on the other hand, requires less accuracy in the manufacture of the structures, but diffraction effects are stronger.
German Offenlegungsschrift 2,133,634, published Jan. 25th, 1973 describes coupled resonance structures which can also be used to improve the flank slope. These resonance structures comprise interference locations disposed perpendicular to the direction of propagation of the surface waves, the distance between adjacent interference points being selected so that a resonator is produced in conjunction with the intermediate portions of surface. Structures of this kind, however, also have to be manufactured very accurately since the accuracy with which the resonant frequency of a resonator can be adjusted depends of the accuracy with which two adjacent interference locations satisfy the condition .lambda./2.
When surface waves propagate, they are attenuated, due mainly to the properties of the piezoelectric material, the surface treatment and the electrodes. These losses, however, can be compensated or the surface waves can even be amplified by using a surface-wave semiconductor amplifier as described e.g. in the article "Surface Wave Delay Line Amplifiers" by Kenneth M. Lakin and H. J. Shaw in "IEEE Transactions on Microwave Theory and Technique," Vol. MTT-17, No. 11, November 1969, Pages 912-920. In this case, the amplifier is substantially either a piezoelectric semiconductive substrate connected to a source of voltage and inserted in the filter-element surface between the input and the output transducer, or a second element made of semiconductive material, connected to a source of voltage, and disposed at a short distance from the filter element made of piezoelectric material. However, the properties of the aforementioned surface-wave amplifiers are discussed only with reference to simple surface-wave filters which do not have any interference locations on the filter-element surface.